Strain-sensitive spectral features are portions of optical absorption lines that shift, split, or broaden under the influence of an applied ultrasonic field. One class of strain-sensitive spectral features are spectral holes, which consist of narrow depressions or dips in the inhomogeneously broadened absorption lines of centers in solids at low temperatures. Persistent spectral hole formation, or hole burning, has been produced by photochemical processes in organic and inorganic systems as well as by nonphotochemical or photophysical mechanisms in glasses and crystals. In addition to providing important basic information about guest-light and guest-host interactions, persistent spectral holes can be used to store digital data in a frequency domain optical storage system, described in U.S. Pat. No. 4,101,976. Because holes burned on nanosecond time scales are usually shallow (see for example, Romagnoli, et al., Journal of the Optical Society of America B: Optical Physics, Vol. 1, 341 (1984)), high sensitivity methods for the detection and/or observation of spectral holes at a high speed are essential.
A variety of optical techniques are available for the detection of specific spectral features including spectral holes. The most elementary methods include transmission spectroscopy and fluorescence excitation with narrow band tunable lasers. These techniques suffer from the limitation that they do not have zero background, so that detection of shallow holes is limited by the ability to accurately remove large baselines. Other detection techniques have been devised for the detection of weak spectral features which utilize indirect, external modulation to achieve high sensitivity and/or zero background. For example, frequency modulation (FM) spectroscopy (described in U.S. Pat. No. 4,297,035) phase-modulates a probing laser beam before passing the beam through the absorbing sample. In fact, FM spectroscopy has been applied to the detection of spectral holes with zero background. Because the signal appears as amplitude modulation of a laser beam at MHz frequencies where laser noise fluctuations are only due to shot-noise, this method can show quantum-limited sensitivity. However, FM spectroscopy requires an electro-optic modulator for the production of frequency modulated light to probe the sample transmission. In addition, residual amplitude modulation hampers the application of FM spectroscopy in some cases.
External modulation methods like FM spectroscopy, including amplitude modulation, wavelength modulation, frequency modulated polarization spectroscopy (described in Ser. No. 06/511593 filed July 7, 1983) and polarization spectroscopy also suffer the following shortcoming. Any perturbation of the carefully prepared probing beam by any optical element in the system other than the sample will produce spurious background signals. The frequency modulated polarization spectroscopy technique overcomes this problem to an extent, but this method is complex and requires anisotropic holes. For example, FM spectroscopy suffers from spurious signals due to any frequency-dependent transmission present in all optical elements between the modulator and the detector other than the sample such as Fabry-Perot resonances in windows, lens coatings, etc. Polarization spectroscopy is sensitive to low frequency laser power fluctuations and background birefringence. The basic fact is that with the external modulation detection methods described above, all frequency-dependent absorptions and dispersions are detected including effects that arise from other reasons than the property of the sample under study.
Ultrasonically modulated electron paramagnetic resonances have been reported in J. Phys. C. Solid State Vol 13, p 865 (1980). In this non-optical technique, effects of 40 KHz strain fields were compared with 40 KHz magnetic fields. Incoherent, non-phase-sensitive ultrasonic modulation of persistent spectral holes has also been reported in Appl. Phys. Letter Vol 43, page 437 (1983). The method used incoherent pulses of ultrasound to modulate persistent holes. The paper emphasized that this technique can be used as a phase-insensitive optical detector for ultrasound in solids.